1. Field of the Invention
This invention relates to a method of exposing and an apparatus therefor. More particularly, the present invention relates to an exposure apparatus of, for example, a slit scanning exposure type and an exposure method advantageously applicable to such apparatus.
2. Related Background Art
Projection type exposure apparatus has been used in manufacturing semiconductor devices, liquid crystal displays and thin-film magnetic heads through a photolithography process, in which patterns of a photomask or a reticle (hereinafter, referred generally to as "reticle") are transferred to the surface of a substrate (wafer, glass plate, etc.) coated with a photosensitive coating.
The conventional projection type exposure apparatus commonly used is a reduction projection type exposure apparatus (stepper) that moves individual shots on the wafer successively to an exposing field of a projection optical system to reproduce pattern images of the reticle on the shots using a photographic step-and-repeat process.
In the typical steppers, a wafer coordinate system is corresponded to a reticle coordinate system (reticle alignment).
Some steppers comprise an alignment microscope of an off-axis type provided at one side surface of the projection optical system to detect a position of the alignment mark (wafer mark) formed as a correspondence to each shot on the wafer. In such a case, the shot on the wafer is determined within the exposing field of the projection optical system according to the position of the associated wafer mark detected on the alignment microscope. Accordingly, so-called base line amount should be obtained previously that represents a distance between a reference point (such as an exposure center) within the exposing field of the projection optical system and a reference point in an observing field of the alignment microscope of the off-axis type.
The reticle alignment and the base line measurement are disclosed in detail in, for example, Japanese Patent Application Laid-Open No. 4-324923 (corresponding to U.S. patent application Ser. No. 872,750 (filed on Apr. 21, 1992), now U.S. Pat. No. 5,243,195).
In recent years, fine patterns for the semiconductor devices require resolution of the projection optical system to be improved. To improve the resolution, exposing light may be shifted to a shorter wavelength or alternatively, the number of openings of the projection optical system may be increased. In any event, it has been difficult to maintain an image quality (such as distortion and image plate deformation) on the entire exposing field with a predetermined accuracy when it is intended to ensure as same exposing field as conventional arts. With this respect, the projection type exposure apparatuses based on so-called slit scanning exposure type have taken a favorable turn.
In the projection type exposure apparatus of the slit scanning exposure type, the reticle and the wafer are scanned relative to and synchronous with a rectangular or arc-shaped illumination field (hereinafter, referred to as a "slit-shaped illumination field") to transfer the patterns of the reticle on the wafer. The slit scanning exposure type thus makes it possible to reduce the exposing field of the projection optical system as compared with the stepper type, provided that the reproduced patterns are equal in area to those reproduced using the stepper type. This may improve the accuracy of the image quality within the exposing field. A six-inch size is dominant for the conventional reticles while a one-fifth factor is dominant as the projection magnification of the projection optical system. At the magnification of one-fifth factor, the six-inch reticle may sometimes be insufficient for recent circuit patterns of the semiconductor device having the increased area. As a result, the projection optical systems should so designed that the projection magnification of the projection optical system is changed to, for example, quarter factors. To comply with requirements for such reproduced patterns having the increased area, the slit scanning exposure type can advantageously be applied.
In this event, the alignment method based on the reticle and wafer coordinate systems used in the conventional steppers may be unfavorable when being applied to the projection type exposure apparatus of the slit scanning type. The projection magnification of quarter factors adversely affects the accuracy of the alignment because the alignment becomes more sensitive to writing errors of the circuit patterns on the reticle.
A technique has been suggested in the above mentioned U.S. Pat. No. 5,243,195 (filed on Apr. 21, 1992) to measure a rotation angle of the reticle by means of measuring simultaneously the amount of displacement of two or more measuring marks rather than moving the wafer stages in the wafer. However, the idea of measuring the rotation angle using the simultaneous measuring of the measuring marks cannot be applied to scanning directions of the projection type exposure apparatus of the slit scanning exposure type. Thus, there is a disadvantage that it is impossible to measure the rotation angle of the reticle and wafer coordinate systems and orthogonal amount of the coordinates of these coordinate systems.
As for the method of measuring the base line amount between the reference position within the exposing field of the projection optical system and the reference position of the alignment system of the off-axis type, the conventional measuring method using a pair or marks on the reticle in the stepper is disadvantageous, when it is applied to the projection type exposure apparatus of the slit scanning exposure type with no modification, in that the writing error of the reticle significantly affects the measurements.